In recent years, attention is focused on FRP, particularly CFRP (Carbon Fiber Reinforced Plastics) as the structural material for the bodies and wings of aircrafts. CFRP may be offered as a composite sheet material based on a combination of titanium alloy, aluminium, and the like.
The composite plate employs a sheet having metal laminated on one face of CFRP (hereinafter, simply referred to as “laminated sheet”). The laminated sheet must have a hole bored to allow a joining member such as a bolt or rivet to be inserted when the laminated sheet is to constitute a body, wing, or the like of an aircraft.
As a tool to bore a hole, the drill proposed for machining FRP disclosed in Japanese Patent Laying-Open No. 5-177420 (PTD 1), for example, may be employed.
The drill disclosed in PTD 1 has the point angle set in multi-stages. The point angle of the outer diameter side is set smaller than the point angle of the inner diameter side. The multi-stage setting causes the spreading action at the outer circumferential side of the bit (outer end side in the outer radial direction) to be reduced, such that generation of a burr can be suppressed at the exit side (egress) of the through hole that is bored.